The electrical system known as a two-wire sensor is a device which has a high impedance when a triggering event (e.g., the presence of an object) is sensed and a low impedance when a triggering event is not sensed. In order to control the operation of an electrical load (e.g., a relay or an input to a programmable controller), the two-wire sensor is connected to an electrical power source in series with the load. Depending upon the presence or absence of a triggering event, the two-wire sensor turns the load "off" or "on." When the load is off (i.e., the two-wire sensor is in the high impedance mode), enough current must pass through the load to operate the sensor. If the current is insufficient, the sensor cannot sense a triggering event and cannot turn on the load in response to the triggering event. However, the current must be sufficiently low to prevent operation of the load. If the load is on (i.e., the two-wire sensor is in the low impedance mode), there must be sufficient power to operate the sensor. If the power is insufficient, the sensor cannot continue to sense the presence or absence of a triggering event. Previous known two-wire sensors fall into one of two broad categories: delayed turn-on circuits and continuous mode circuits.
A typical delayed turn-on circuit is shown in FIG. 1. The delayed turn-on circuit comprises: (1) a voltage supply 200; (2) a load 202; (3) a full wave bridge rectifier 204 (rectifier circuit, which is comprised of diodes 206, 208, 210 and 212); (4) an SCR 214; (5) resistors 216, 222 and 230; (6) a zener diode 218; (7) a capacitor 220; (8) voltage regulators #1 and #2, which are 224 and 228, respectively; (9) transistors 226 and 232; and (10) a sensor 234, all interconnected as shown in FIG. 1.
In the delayed turn-on circuits, an SCR 214 is used as a solid state switch to cause a high and low impedance path for the load current. Just prior to the turn-on or firing of the SCR 214, a transistor 226 (or sometimes a second SCR) is momentarily turned on to charge a capacitor 220. When the voltage across the capacitor 220 attains the breakdown voltage of a zener diode 218 connected in series with the gate of the SCR 214, the SCR 214 is gated into its low impedance mode, and the charge stored on capacitor 220 acts as a storage device for the balance of the AC current cycle in order to power the sensor 234 via voltage regulator #2 228. Each time the AC cycle crosses zero volts, the SCR 214 turns off and does not turn on until the capacitor's charge is replenished. Since the capacitor 220 must be recharged every half cycle, delayed turn-on circuits can only be used with AC voltage inputs.
A typical continuous mode circuit is shown in FIG. 2. The continuous mode circuit comprises: (1) a voltage supply 300; (2) a load 302; (3) a full wave bridge rectifier 304 (which is comprised of diodes 306, 308, 310 and 312); (4) an SCR 314; (5) resistors 316, 318 and 320; (6) diodes 322 and 324; (7) zener diode 326; (8) capacitor 328; (9) transistor 330; and (10) sensor 332, all interconnected as shown in FIG. 2.
The continuous mode circuits use a zener diode 326 and an SCR 314 in series, so that current is supplied from the SCR 314 and zener diode 326 via diode 322 to power the sensor 332 when the SCR 314 is in its low impedance mode. Further, in this state, resistor 320 provides a path for current to charge capacitor 328 and for current to the sensor 332. Diode 324 and zener diode 326 provide adequate voltage regulation.
Continuous mode circuits have at least two disadvantages. First, there must be approximately 6 to 8 volts across the zener diode 326 in order to produce a high enough voltage to operate the sensor's electronics. Second, due to the power dissipated in the zener diode 326 (4 watts at 500 milliamps for 8 volts and 3 watts at 500 milliamps at 6 volts), package size increases, reliability decreases, and load current switching capability is limited.
Accordingly, it is a primary object of the present invention to provide a two-wire power control and sensing circuit which can switch AC or DC power, operate over wide supply voltage ranges (20 to 260 volts AC or DC) and switch 500 milliamps or more.
Another object of the present invention is to provide a two-wire sensor having a small package size and minimal power dissipation.
Yet another object of the present invention is to provide a two-wire sensor that can replenish a voltage supply for a sensing function whenever required.